From the Super Mario Wiki
Bowser??? is a boss in the game Paper Mario. It is a large, mechanical effigy of Bowser controlled by the Koopa Bros. It is used by them in order to fool Mario into thinking that it is indeed the Koopa King. Bowser??? only possesses one attack, in which the Koopa Bros. roll the mech over to Mario and hit him with its hand. Throughout the battle, the Koopa Bros. also taunt Mario from inside Bowser??? after attacking him.
Bowser??? appears to be made out of cloth for the skin and shell, buttons for the eyes, and cardboard for its arms and legs. Under the feet is a cart that allows it to move. The fact that Bowser??? is false is revealed by its overall appearance, and is proven when the Koopa Bros. are conversing from inside of it during the battle. Before the battle, the Koopa Bros. are arguing that someone is "being a hog" and should "move over", which suggests there are multiple occupants inside, in a manner akin to a Chinese marching dragon or pantomime horse costume; in fact, the theme song that plays in this fight is called "Trojan Bowser" which is a reference to the Trojan Horse in Greek mythology. After being defeated, Bowser??? breaks into pieces and falls into the prison cell used to hold the Bob-ombs, followed by the Koopa Bros. emerging to fight Mario themselves. A picture of Princess Peach can be seen inside Bowser???'s shell when it is defeated, and more clearly after the Koopa Bros. are defeated.
In the ending parade of Paper Mario, Bowser???'s remains have been transformed into a float for the Koopa Bros.
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